Ketogenic Diets and Low Carb Diets: The Effects on the Intestinal Microbiota. A Narrative Review

Dietary patterns significantly influence the gut microbiota composition, which in turn affects many host homeostasis and biological functions. This narrative review examines the effects of ketogenic diets (KDs) and low-carbohydrate diets (LCDs) on the human gut microbiota, highlighting distinct microbial shifts associated with carbohydrate restriction. A total of 10 studies were analyzed, encompassing 346 healthy adults aged 18-74 years, including normal-weight, overweight, and obese individuals without any other diagnosed pathological conditions, with BMI ranging from 20 to 48.48 kg/m2. The available evidence suggests that reductions in carbohydrate intake are associated with taxonomic changes at multiple levels of the gut microbiota. Phylum-level responses, particularly involving Bacteroidetes and Actinobacteria were heterogeneous and appeared to depend on the degree and type of carbohydrate restriction, with some studies reporting opposing trends, especially under very-low-carbohydrate ketogenic protocols. Overall, the variability among the studies highlights the influence of dietary composition, intervention duration, and methodological differences on microbiota outcomes. These findings underscore the need for standardized approaches and long-term human studies to clarify the implications of ketogenic and low-carbohydrate diets for the gut microbiota composition and host health.